When flexographic printing plates are mounted on the cylindrical drum of a printing press, a non-linear image distortion is created on the outer (image bearing) surface of the plate. Such a distortion is caused by the stretching of the plate when it is wrapped around the cylindrical drum. This stretching effect is illustrated in FIGS. 1a and 1b. This is not to be confused with the uniform stretching caused simply by bending any materials into cylindrical shape, also known as the "k factor" in flexography.
FIGS. 1a and 1b depict typical flexographic plates. Each plate 10 consists of a substrate 12 and an elastomeric, image carrying layer 11. On a flexographic plate, the elastomeric material 11 contains image features 14 which are cut into its exterior 17. The depth and density of the features 14 are dependent on the image data. The depth and density determine the effective average thickness of the elastomeric layer 11. In the following disclosure, the phrases "depth and density" and "effective thickness" are used interchangeably. A relatively high "depth and density" corresponds to a relatively low "effective thickness" and a relatively low "depth and density" corresponds to a relatively high "effective thickness".
As shown in FIG. 1b, when a plate 10 is wrapped around the cylindrical drum 15 of a printing press (not shown), stretching occurs on its outer, image bearing surface 17 and compression occurs on its inner, substrate surface 16. On the neutral plane (represented by hash line 13) no stretching or compression occurs. The neutral plane 13 is located a distance t.sub.2 from the substrate side of the plate 16 and a distance t.sub.1 from the image side of the plate 17. Typically, the substrate 12 is made of a material which is less pliant than that of the elastomeric image carrying area 11; consequently, t.sub.2 is less than t.sub.1. However, as can be seen by comparing FIG. 1a to FIG. 1b, the depth and density of image reliefs 14 (or effective thickness of the elastomeric layer) also affects the location of the neutral plane 13. A lower effective thickness of elastomer results in a neutral plane 13 which is located closer to the substrate side 16 of the plate (i.e. t.sub.1 increases and t.sub.2 decreases with the depth and density of image gravures 14).
When the plate 10 is wrapped around the cylindrical drum 15 of a printing press, stretching occurs on its outer, image bearing surface 17 and compression occurs on its inner surface 16. Because of this stretching, the printed image will exhibit a distortion proportional to the stretching of the image bearing surface 17. Obviously, this distortion degrades the quality of the printed image. As such, there is required in the art a method of minimizing the distortion caused by stretching of the flexographic printing plate as it is mounted on the cylindrical drum of a printing press. This non-linear distortion is particularly apparent when different plates have to register together, such as when printing with more than one color.
In prior art techniques of imaging a flexographic plate 10, the image imparted onto the plate 10 may be reduced by a constant amount so as to compensate for this stretching effect. However, constant compensation over the entire plate 10 does not account for localized deviations in the depth of the neutral plane 13 and the localized distortion of the image bearing surface 17 caused by variations in the effective thickness of the elastomer. This is illustrated by a linear scale 18 showing the effect of local distortion. As such, there is a need in the art for a compensation technique for the imaging of flexographic plates, which minimizes the distortion caused by stretching and accounts for localized variations in the depth and density of image relief.
It is an object of this invention to create a compensation scheme wherein the aforementioned stretching distortion can be reduced, so as to improve the quality of the printed images from a flexographic printing press.
It is a further object of this invention to create a compensation scheme, which predicts localized distortion at various parts of the image and accounts for localized variations in relief depth and density on the image bearing surface of the plate.